1. Field of the Invention
The invention relates to antenna modules for installation under dielectric covers and more particularly to antenna modules for use in motor vehicle cab structures made of dielectric material such as fiberglass.
2. Related Art
Presently motor vehicles such as cars, trucks, recreational vehicles and the like use several antennas for such purposes as cellular telephones, CB, global positioning system (GPS) as well as the standard AM/FM radio. Typically, a separate antenna mounted external to the body of the vehicle is provided for each such system. This proliferation of antennae is attended by special problems such as finding an appropriate mounting position for non-interfering operation as well as such inconveniences as high speed antennae noise or "whistle". Attempts have been made in the prior art to avoid the external antennas and incorporating antennas into window panes and roof panels and the like.
Non-conducting materials, e.g. , fiberglass, has been used for some time in the construction of cars and especially in the construction of truck cabs. The use of such a dielectric material presents a problem for antenna designers since most antennae require the ground plane provided by the metallic vehicle body for efficiency of operation.
One prior patent, U.S. Pat. No. 4,737,795 issued Apr. 12, 1988 describes an AM/FM antenna in the form of a slot antenna formed in a horizontal sheet of conducting material and installed under a non-conductive roof portion inserted in a metallic vehicle roof. Such slot antennas are relatively expensive and for optimum reception preferably employ separate feed lines for AM and FM, adding further to the expense of the antenna installation. While the use of such antenna has been proposed for CB and cellular telephone, it is not clear that such a slot antenna will work effectively for those purposes,
In another known arrangement, described in U.S. Pat. No. 4,821,040 issued Apr. 11, 1989, a circularly shaped slot antenna spaced above a conductive reference surface, is mounted under the plastic roof of a vehicle for operation in the mobile radio frequency range. An annular resonant cavity is defined between a disk and a conductive reference surface. The disk is supported on a post which may be up to three-quarter inch in height providing an overall structure of significant thickness between the roof and the headliner of an automobile. The antenna consists of a number of parts which must be separately machined or die stamped or the like. Such an antenna will be relatively expensive to manufacture.
In another known arrangement, shown in U.S. Pat. No. 5,124,714 issued Jun. 23, 1992 a planar slot antenna is disclosed having an inner and outer conducting surface for AM/FM reception and a closed-circle slotted antenna formed in the inner conductive surface for use in the telephone frequency range. A particular disadvantage of that antenna is that the output from one slot antenna is interrupted when communications are received by the other slot antenna.
A significant problem of the industry is that no efficient and relatively inexpensive antenna arrangement is available for use with vehicle cab structure made of a dielectric material.